One of the main applications of photoplethysmographic (PPG) biosensors is measuring blood oxygen saturation. The commercial device based upon this technology is a pulse oximeter. Pulse oximetry may be used to quantify various blood flow characteristics including arterial oxygen saturation, the volume of blood pulsation carried to the tissues, and the heart rate. A pulse oximeter typically uses two light emitting diodes (LEDs) with wavelengths in red and infrared regions which emit light at a measurement site on a human body. A photodetector captures the transmitted or reflected light. The analog front-end hardware uses a time multiplexed approach with three phases: (i) an activated red LED phase, (ii) an activated infrared LED phase, and (iii) a dark phase. A transimpedance amplifier amplifies the current generated in the photodetector due to optical density during active phases and provides a voltage signal. The voltage signal is filtered, amplified, and sampled with an analog-to-digital converter for further processing.
At the measurement site (e.g. finger, ear, forehead, nasal), the tissue slightly expands during each heart beat as blood enters via arteries during systole. Then, the site contracts as blood leaves during diastole. As a result, the path length of the light will periodically change. Absorption is proportional to the optical path length according to the Lambert law of optical density and the blood volume change will be reflected in the output of the photodetector. Referring to FIG. 1, the light absorbance at sensor site 100 has three main signal components: a nonpulsatile component 101 originated from nonpulsatile blood, tissue pigmentation (e.g. skin, bone, muscle), which results in a direct current (DC) signal in the photodetector, a weaker signal 102 caused by blood return in veins, and a dominant alternating current (AC) 103 caused by blood volume change in arteries.
Light absorption is also a function of the hemoglobin concentration in blood and a “ratio of ratios” technique defines R as an intensity-independent parameter. When R is large, the blood saturation is low and vice versa. An empirical linear calibration curve relates the measured R to oxygen saturation where R is mathematically expressed as:
                    R        =                                            AC              R                        /                          DC              R                                                          AC              IR                        /                          DC              IR                                                          Eq        .                                  ⁢        1            where, AC and DC are the ac and dc power of the PPG signals and indices R and IR refer to red and infrared, respectively. The existence of the venous component which has the information of blood return to the heart has been verified with different techniques, including imaging-based techniques, such as phase contrast magnetic resonance angiography.
PPG sensors also capture blood volume change in veins. The sensor's output AC current is a mixture of arterial and venous signals. Existence of the venous component causes the AC power of the PPG signals to change which leads to inaccuracy and false alarms in readings such as oxygen saturation level. In clinical settings, the venous signal has a significant effect on the shape of PPG signal and the SpO2 readings. For example, a commercial forehead probe called The Nellcor™ manufactured by Covidien LP of Mansfield, Mass. needs to be placed above the eyebrow using a self-adhesive bandage. However, this forehead sensor is limited to low saturation readings. In hospital settings using finger, forehead, and ear sensors, a complex waveform can be created by the prominent venous component. However, the power of the venous component signal differs in various sensor sites, leading to inaccurate readings. This inaccuracy is confirmed by applying a pressure dressing to the measurement site, which weakens the effect of the venous signal on the PPG signal.
As seen, venous pulsation remains one of the sources of inaccuracy in various applications of PPG sensors such as arterial oxygen saturation. Researchers have collected clinical data on effect of venous signal/pulsation on PPG signal or pulse oximetry readings. Generally, researchers have observed occurrences of venous signal pulsation on PPG signals and attempted to understand how such pulsation affects the oxygen saturation readings. The main experimental solution suggested has been control of pressure on the sensor during signal acquisition.
The prior art has attempted to address these problems associated with venous signal pulsation with limited success. For example, the system for pulse oximetry from Covidien defines and adjusts the sensitivity level of a PPG sensor. The sensitivity level is adjusted based on the location of the measurement displayed to the user. The system also compares the current R value to historical R values. The historical value may include measurements from the same patient or an average value from other patients. However, in the Covidian system, once a prominent venous is detected, the reading is stopped or ignored. This is a problem because, in many applications, such as under anesthesia during surgery, the intermittent and temporary changes of the oxygen saturation need to be constantly monitored which cannot occur if the readings are stopped.
Other methods of the prior art include extracting information from venous pulsation of the PPG signal by adding a mechanical vibrator to the sensing part. The mechanical vibrator works as an actuator that creates an external artificial perturbation close to the PPG sensor. A pressure transducer is attached at a first site and applies a drive signal at a predetermined frequency. The drive signal causes a series of pulsations of a predetermined magnitude in the veins. Then, variations in blood volume in veins are captured by an optical sensor and used to estimate venous oxygen saturation. However, in order for the system to function, an artificial pulsation is needed from a mechanical source. Since this pulsation is artificial, it does not provide any insight into the actual blood return and natural venous functionality.
Based on the negative role of venous pulsation on signal accuracy, there is a need in the art for a system and method for extracting venous pulsation from PPG sensors. There is a need in the art for a system and method that addresses the inaccuracy in measurement of parameters, such as SpO2, by extracting a high-quality venous signal. There is a further need in the art for a system and method that uses the extracted venous signal on PPG, by extracting other medically relevant information, such as respiration rate.